101 years ago, physicists made a critical discovery we still don’t understand

Nobel laureate Otto Hahn is credited with the discovery of nuclear fission† Fission is one of the most important discoveries of the 20th century, but Hahn regarded something else as his property best scientific work†

In 1921, he was studying radioactivity at the Kaiser Wilhelm Institute for Chemistry in Berlin, Germany, when he noticed something he couldn’t explain. One of the elements he worked with was that he didn’t behave like that should have† Hahn had unwittingly discovered the first nuclear isomer, an atomic nucleus whose protons and neutrons are arranged differently from the element’s ordinary form, giving it unusual properties. It took another 15 years of discoveries in nuclear physics to explain Hahn’s observations.

We are two professors of nuclear physics studying rare nuclei, including nuclear isomers.

The most common place to find isomers is within stars, where they play a role in the nuclear reactions that create new elements† In recent years, researchers have begun to explore how isomers can be used for the benefit of humanity. they are already used in medicine and could one day offer powerful options for energy storage in the form of nuclear batteries†

On the hunt for radioactive isotopes

In the early 1900s, scientists were on the hunt for new radioactive elements. An element is considered radioactive if it spontaneously releases particles in a process called radioactive decay† When this happens, the element is transformed into another element over time.

At the time, scientists relied on three criteria to discover and describe a new radioactive element. One was to look at chemical properties – how the new element reacts with other substances. They also measured the type and energy of the particles released during the radioactive decay. Finally, they would measure how quickly an element decayed. Decay rates are described using the term half-life, which is the amount of time it takes for half of the original radioactive element to decay into something else.

By the 1920s, physicists had discovered some radioactive substances with identical chemical properties but different half-lives. These are called isotopes. Isotopes are different versions of the same element with the same number of protons in their nucleus, but different numbers of neutrons.

Uranium is a radioactive element with many isotopes, two of which occur naturally on Earth. These natural uranium isotopes decay into the element thorium, which in turn decays into protactinium, and each has its own isotopes. Hahn and his colleague Lise Meitner were the first to discover and identify many different isotopes resulting from the decay of the element uranium.

All of the isotopes they studied behaved as expected, except for one. This isotope was found to have the same properties as one of the others, but the half-life was longer. This made no sense, since Hahn and Meitner had put all known isotopes of uranium in a neat classification and there were no empty spaces for a new isotope. They called this substance ‘uranium Z’.

The radioactive signal of uranium Z was about 500 times weaker than the radioactivity of the other isotopes in the sample, so Hahn decided to confirm his observations by using more material. He chemically purchased and separated uranium from 220 pounds (100 kilograms) of highly toxic and rare uranium salt. The surprising result of this second, more precise experiment suggested that the mysterious uranium Z, now known as protactinium-234, was an already known isotope, but with a very different half-life. This was the first case of an isotope with two different half-lives. Hahn published his discovery of the first nuclear isomereven though he couldn’t quite explain it.

Neutrons complete the story

At the time of Hahn’s experiments in the 1920s, scientists still thought of atoms as a clump of protons surrounded by an equal number of electrons. It wasn’t until 1932 that James Chadwick suggested that there might also be a third particle – neutrons part of the core†

With this new information, physicists could immediately explain isotopes — they are nuclei with the same number of protons and different numbers of neutrons. With this knowledge, the scientific community finally had the tools to understand uranium Z.

in 1936 Carl Friedrich von Weizsäcker suggested: that two different substances can have the same number of protons and neutrons in their nuclei, but in different arrangements and with different half-lives. The arrangement of protons and neutrons that results in the lowest energy is the most stable material and is called the ground state. Arrangements that result in less stable, higher energies of an isotope are called isomeric states.

In the beginning, nuclear isomers were only useful in the scientific community as a means of understanding how nuclei behave. But once you understand the properties of an isomer, you may wonder how they can be used.

Isomers in Medicine and Astronomy

Isomers have important applications in medicine and are used in tens of millions of diagnostic procedures each year. Because isomers undergo radioactive decay, special cameras can track them as they move through the body.

For example, technetium-99m is an isomer of technetium-99. When the isomer decays, it emits photons. Using photon detectors, doctors can track how technetium-99m moves throughout the body and make pictures of the heart, brain, lungs and other critical organs to help diagnose diseases, including cancer. Radioactive elements and isotopes are normally dangerous because they emit charged particles that damage body tissues. Isomers like technetium are safe for medical use because they emit only a single, harmless photon at a time and nothing else as they decay.

Isomers are also important in astronomy and astrophysics. Stars are fed by the energy released during nuclear reactions. Since isomers are present in stars, nuclear reactions are different than if the material were in its ground state. This makes the study of isomers critical to understanding how stars produce all the elements in the universe.

Isomers in the future

A century after Hahn first discovered isomers, scientists are still discover new isomers using powerful research facilities around the world, including the Rare Isotope Ray Facility at Michigan State University. This facility came online in May 2022 and we hope it will unlock more than 1,000 new isotopes and isomers.

Scientists are also investigating whether nuclear isomers can be used to build the most accurate clock in the world or that isomers may one day form the basis for the next generation of batteries† More than 100 years after the discovery of a small anomaly in uranium salt, scientists are still searching for new isomers and are just beginning to reveal the full potential of these fascinating pieces of physics.

This article was originally published on The conversation by means of Artemis Spyrou at Michigan State University and Dennis Mücher at the University of Guelph† Read the original article here†